My training buddy just complained during our gymnastics workout today that his A/C joint was bothering him ever since he did weighted (bar) dips the other day.

I believe that for those who have trouble maintaining any semblance of scapular depression in general during dips (or don't know to not let their shoulders go up into their ears as soon as they break the elbows to descend), that weighted dips really invite issues in the A/C joint.